Focusing magnet for long electron beams



June 7, 1960 M. MULLER 2,940,020

FOCUSING MAGNET FOR LONG ELECTRON BEAMS Filed Feb. 16, 1956 Fig. 6

Fig. 5

Fig. 5a

INYENTOR M. MULLER BY Mm;

ATTORNEY United States Patent 2,940,020 FOCUSING MAGNET FOR LONG ELECTRON BEAMS Martin Miiller, Pforzhelm, Germany, asslgnor to Inter national Standard Electric-Corporation, New York, N.Y., a corporation of Delaware This invention relates to focusing magnet structures for long electron beams.

In the production, amplification and modulation of I oscillations having a very high frequency electron tubes are used in which there is effected an exchange of energy between an electromagnetic wave and an electron beam of focused electrons of substantial length in proportion to the cross-section thereof. Such types of tube are, for example, the multi-cavity or multi-catcher klystron and the traveling wave tube. The space-charge constants U3/2 (J=beam current intensity, U=voltage velocity), which are required for such devices, are much higher than, for example, in cathode-ray tubes and necessitate a magnetic beam guidance (focusing) over almost the entire length of the beam. The necessary magnetic force increases with J/ U, and the direction of the lines of flux is required to coincide with the direction of the beam if the beam is not to deviate from the desired straight track. This is particularly necessary in the aforementioned types of tubes, since there are usually present very small diaphragm apertures which often must not be hit by the electrons.

The focusing magnetic field may be produced either by electromagnet coils or by permanent magnets. As is well known, permanent magnets are characterized by having less weight and also have the advantage of requiring no magnetising forces and consequently no direct current supply systems.

The field produced by coils, due to the geometry of the winding may be largely determined and may be represented relatively homogeneously, but the field produced by a permanent magnet depends to a large extent on the leakage values of the material employed. Consequently, even with a regular geometrical structure, the resulting magnetic field will exhibit irregularities. Hence there will appear irregular cross-field components 7 coincidence of the longitudinal field with the electron tube inside the arrangement is prevented from being estab-' lished. V

The individual sections which introduce such irregularities cause a deflection of the electron beam out of the desired direction, thereby causing substantial beam current losses and, consequently, a diminution in the amplification-and output of thetube. Furthermore, these, deflecti'ons may result in unduly high heating of the delay line in the tube, which may easily cause the destruction thereof.

For .elirninating these disadvantages, arrangements have been proposed in which, by means of soft iron mag netic homogenizing sheet members arranged between the elements of the magnet,.there is attempted to achieve a more uniform distribution of the resulting field. In these arrangements, however, the field had to be purposely reduced to such a value lying far below the magnetic field strength which actually could be produced for reasons 2,940,020 Patented June 7, 1960 that will be apparent in the following description. For this reason permanent magnets could hitherto only be employed with tubes of relatively low gain and output.

In the accompanying drawings:

Figs. 1 to 3 are diagrams used to explain the state of prior art;

Fig. 4 shows a diagram relating to the mode of operation of the invention;

Fig. 5 shows a focusing magnet structure according to the invention, and

Fig. 6 shows a sectional elevation taken on line C-C' of the arrangement as shown in Fig. 5.

One conventional arrangement is shown in Fig. 1 of the accompanying drawing. Around the tube R there is arranged a magnet structure M, composed of individual magnet sections T, T which produces a longitudinally extending field. The individual magnet sections are magnetized in the axial direction --N, S and are separated from each other by means of homogenizing sheet members B.

A transverse field will result when, for example, the poles arranged above and below the tube, at A and A indicating the section line, have different magnetic potentials, and thus have an undesired magnetic difference of potential with respect to one another. This difference of magnetic potential is supposed to be compensated by the adjoining sheet member B serving to short-circuit the common pole surface A, A of the respective section with a high ferromagnetic permeability.

Fig. 2 is a sectional elevation through the arrangement according to Fig. 1 taken in line A-A'. T and T denote the upper and the lower partial magnets, while r denotes the central aperture of the sheet members B for receiving the tube R.

Experiments have shown that the desired function of the sheet members B can only be accomplished when the whereby the axial magnet is operated in a permanent state, viz. in a state which, after the magnctisation, is substantially weakened. When fully utilizing the field strength of the magnet, however, the internal resistance of the source of magnetic voltage is so low that the flux flowing through the sheet members B easily magnetises them up to the saturation point, thereby limiting the compensation of flux.

The measure of providing continuous sheet members therefore, only brings about a slight improvement of the magnetic field, because the source of disturbing voltage cannot be sufficiently short-circuited. In'Fig. 3 there is shown the magnetic potential P between the poles A and A, which may appear in the case of different properties of the material employed. Curve a in Fig.. 3 shows the spacial course when employing no homogenizing sheet, while curve b indicates the potential along theabove-mentioned sheet member. The magnetic voltages corresponding to the difference of potential within the total range D with respect to both cases are denoted by V and V respectively. It will be clearly seen that the obtained voltage V is only slightly smaller than-the corresponding voltage V,,.

Now the field strength H, at any point, as is well known, equals the gradient of the potential, hence the only transverse field strength H of interest simply equals the derivation of the potential curve which, in both cases and within the total range D, is constant. The weakening of the field obtained with the aid of the conventional homogenizing sheet members, is also evidenced by the reduced slope of the curve b. There exists, therefore, the problem of further weakening the transverse field, in particular within the mean partial range of D and, if possible, to cause it to disappear entirely.

In accordance with the present invention this is accomplished in that the homogenizing sheet members are separated from the magnetic source of disturbance by means of a non-ferromagnetic gap extending in the circumferential direction. By means of .this arrangement it is possible to practically eliminate the magnetic difference of potential in the sheet within the gap D. The still-existing transverse difference of magnetic potential is thus eifectivelyremoved fromthe center range, in which it would be likely to affect the electron beam.

The .-non-.ferromagnetic gap extending in the circumferential direction may consist of any suitable material. Since the outside portions of the sheet members, as hitherto, cause only a slight weakening of the magnetic transverse difference of potential,-.the gap now results in a fundamentally different potential distribution inside itsrange. Due to its extraordinary high magnetic resistance the gap prevents the passage of a measurable transverse flux In, by practically taking up the entire magnetic voltage drop V itself. Since the magnetic resistance km of the inner sheet member has remained unchanged, the magnetic difference of potential .must have become .disappearingly small there because Accordingly, in the inner range 'd also .the transverse field strength Hat;

is 'sosma'll that it can be neglected and a sidewise interaction upon the electron beam is precluded.

For appreciating the underlying principle of the invention it should be sufficient tostudy Fig. 1 and Fig. 2 of the drawings. One only needs'to imagine that the homogenizing sheet members B as shown in these-figures, besides their central aperture r, comprise a gap of a extends over the central aperture through which the electron beam proceeds. Hence the invention consists in the provision of a magnet structure comprising homogenizing sheet members surrounding the electron tube, which are not in magnetic contact with the sources of magnetism, for

example, the permanent magnet sections.

The precise shape of thehomogenizing sheet members is of secondary importance. By way .ofrexample, they may :eitherh'ave a circular, loval, square or irregular configuration. The same, ofcourse, also applied to the nonferromagnetic airgap, the embodiment of which should 'I be adapted to the electrical and mechanical conditions required to be met. .Especiallythe width'of -the gapmay vary it, by way of this measure, there can be obtained a better physical cifect in any particular application. The longitudinal field, which effects the focusing of the beam,

' however, will be partially weakened within the small non-magnetic material extending in the circumferential V direction. "Hence they should be imagined as being-interrupted, eg. by'means of an inserted ring of brass serving, because of its high magnetic resistance, to separate the inner disk with the diameter a (Fig. 4) .from the outer sheet member. When comparing Fig. '4 with Fig. '3 the new course of the transverse difference of magneticzpotention P (P -P between the two partial magnets T and T, originatingfrom the same source of disturbance,- will be observed and two very different curves c and 2 will be noted. The dashed curve 0 represents thestill existing linear course of the potentialithat is formed outside the gap, hence in the cut-off outer portion ofthe sheet member B,,. As already described 'hereinbefore, the latter acts as a flux compensating sheet memberbetween the two poles of the magnet. Theinclination of curve 0 corresponds to a mean value between the curves at and b of Fig. 3, because the sheet member, on account of being cut-ofi from the inner disk has slightly .lost some ofits conductivity. The curve e now represents the course of the magnetic potential across the gaps S and within the center range, and exhibits the desirable property of homogenizing the field to an almost ideal extent. .It will be obvious that the central aperture r must be just as field-free as the field-free inner :plate havingthe diameter d which serves to short-circuit the high resistance of the aperture. This will be understood when it is appreciated that also the gap at the .two points of contact (p and p with the outer flux compensating sheet member, must necessarily have the potential thereof. e

In the gap itself, however, there appears a strong potential drop, whereas the middle section of the curve e will extend practically horizontally. The ascent? there-' of, hence the field strength, is times smaller because the very weak transverse flux whichfiows across thegap,

also encountered there a n times greater-conductivity. Therewith the inner portion of the sheet provides a practically field-free space which, at the same time, also magnet sections.

range of the thin sheet members, but will not be .disturbed' in its direction as "longas'the sheet members ex-- tend exactly perpendicular in relation to the 'fi'eld' axis.

One particular example 'o-fan embodiment of the invention which carefully complies with the aforementioned requirements :is shownin Fig. '5. .In this casethe outer flux compensation sheet :members '13,, are constituted by separate elements, :while the homogenizing "sheet -members 2B are separated from each other by means of spacersZ which are threaded on a non-ferromagnetic supporting means M. In Fig. So there is shown in section one such spacer, consisting e.g. of aluminum and exhibiting plane-parallel outer surfaces, the other con figuration not being critical. .It will bev seen furthermore from Fig. v5 that the number of. inner homogenizingsheet membcrsB used may be independent of the :number of Hence the outer magnet structure no longer needs 'to beconstructed of numerous very'short magnet sections. With respect to the dimensions of the homogenizing sheet members B it has been found that the width of the ring spacers'Z has to be chosen as large as possible, at least just as large as the distance between two adjacent sheet. members, if it. is required to obtain a particularly .good homogenization of the inner field. The free inner space which in the case of Fig. 4 is slightly smaller than with the arrangement of Fig. 1, depends 'on the dimensions of the electron tube to be employed.

.Fig. .6 is a section taken on line C-C through the i structure of Fig. 5. The magnet components E, arranged between the magnets T and T, are likewise barshaped magnets which are magnetised in the direction of the tube axis and serve to increase the longitudinal fieldstrength.

As has already been pointed out, 'the shape of the sheet member's B may be chosen :at will. Thesame .applies to the shape "of the non-ferromagnetic gap, the width of which'may either remain unchanged .or, for .example, with respect to horizontal or vertically acting influences along-its extension, may be varied at will.

In one embodiment of the invention, even when 'employing permanent magnets with considerable leakage, there could be traced no measurable transverse field strength. When compared with the conventional arrangements employing continuous homogenizing-sheet members, the invention has the added advantage that .by the employment of such inner members projecting into the range of the outer field of the magnetstructure, the

inner field cannot be influenced. The invention offers The aforementioned exemplified embodiments are givenfor enabling a proper understanding of the invention but they, :in no way, :are intended to limit the scope of the invention or its range of practical application.

What is claimed is:

1. Magnet structure for focusing an elongated electron beam, comprising a plurality of magnetic field-producing elements located adjacent each other along the axis of the beam, said field producing elements having their opposing surfaces of opposite polarity to produce a longitudinal magnetic field extending along the trajectory axis of said beam, first and second field-homogenizing means of high magnetic permeability and extending transversely to the axis of said beam, each of said first field-homogenizing means being positioned between two adjacent magnetic field producing elements, said second fieldhomogenizing means being positioned between said field producing elements and the axis of said beam, certain of said second homogenizing means being aligned respectively with certain of said first field-homogenizing means, and said first field-homogenizing means transversely spaced from said second field-homogenizing means to form a non-ferromagnetic gap.

2. Magnet structure according to claim 1 further comprising further field-homogenizing means, each of said further field-homogenizing means located between two adjacent of said second field-homogenizing means, said further homogenizing means being of high magnetic permeability and located between said field-producing elements and said axis to define an additional non-ferromagnetic gap between said further means and said field-producing elements.

3. Magnet structure according to claim 1 in which said second field-homogenizing means is in the form of an annulus and said first field homogenizing means is also in the form of an annulus surrounding the said second field-homogenizing means but having an inner diameter which is larger than .the outer diameter of said second field-homogenizing means to provide said nonferromagnetic gap.

4. Magnet structure according to claim 3 in which the said second of said field-homogenizing means are threaded on an axially supporting tubular non-magnetic member and spaced along said tubular member by intervening non-magnetic spacers.

5. Magnet structure according to claim 1 in which each of said field-producing elements is a permanent magnet each polarized in the same way along the said axis, and in which said first field of each homogenizing means adjacent the said magnets is sandwiched between the adjacent ends of a pair of such magnets.

References Cited in the file of this patent UNITED STATES PATENTS 2,666,163 Clogston Ian. 12, 1954 2,791,718 Glass May 7, 1957 2,797,360 Rogers et al. June 25, 1957 2,798,203 Robertson July 2, 1957 FOREIGN PATENTS 742,070 Great Britain Dec. 21, 1955 

